In general, next-generation communication systems are developing in pursuit of a system capable of providing a service allowing high-speed large-capacity data transmission and reception to Mobile Stations (MSs). However, different from wire channel environments, wireless channel environments of mobile communication systems undergo occurrence of inevitable errors, which cause loss of information, due to various factors, such as multi-path interference, shadowing, electric wave attenuation, time-varying noise, and interference and fading.
The loss of information may cause severe distortion of an actual transmission signal, thereby degrading the entire mobile communication system performance. Therefore, a diversity scheme is used in order to eliminate the instability of communication due to the fading. In large, the diversity scheme can be classified into a time diversity scheme, a frequency diversity scheme, and an antenna diversity scheme, that is, a space diversity scheme.
The antenna diversity scheme is a scheme using multiple antennas and can be classified into a reception antenna diversity scheme using multiple reception antennas, a transmission antenna diversity scheme using multiple transmission antennas, and a Multiple Input Multiple Output (MIMO) diversity scheme using multiple reception antennas and multiple transmission antennas. The MIMO scheme is a kind of Space-Time Coding (STC) scheme. According to the STC scheme, signals encoded according to a predetermined encoding scheme are transmitted through multiple transmission antennas, so that an encoding scheme of a time domain is expanded to a space domain, so as to achieve a lower error rate.
A representative of such MIMO schemes is a Double Space Time Transmit Diversity (DSTTD) scheme. In a mobile communication system using the DSTTD scheme (DSTTD mobile communication system), two independent data streams are encoded by using two Space Time Block Coding (STBC) encoders before transmission, so as to simultaneously acquire both multiplexing and diversity gain. In the DSTTD mobile communication system, because multiple antennas are used, the spatial correlation between antennas has an influence on the entire system performance. Therefore, in the DSTTD mobile communication system, antenna grouping is performed by using an antenna grouping matrix in order to solve the problem of spatial correlation. Hereinafter, a method for antenna grouping using an antenna grouping matrix will be described with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram illustrating a structure of a signal transmission apparatus of a typical DSTTD mobile communication system.
Referring to FIG. 1, the signal transmission apparatus includes a serial to parallel (S/P) converter 111, STBC encoders 113-1 and 113-2, an antenna grouping unit 115, and transmission antennas 117-1, 117-2, 117-3, and 117-4.
First, when a transmission data stream X to be transmitted occurs, the transmission data stream X is input to the S/P converter 111. Then, the S/P converter 111 converts the transmission data stream X into two parallel sub-transmission data streams x1x2 and x3x4, and then outputs the sub-transmission data stream x1x2 to the STBC encoder #1 113-1 and the sub-transmission data stream x3x4 to the STBC encoder #2 113-2.
The STBC encoder #1 113-1 encodes the sub-transmission data stream x1x2 according to the STBC scheme and then outputs the encoded sub-transmission data stream x1x2 to the antenna grouping unit 115. Further, the STBC encoder #1 113-2 encodes the sub-transmission data stream x3x4 according to the STBC scheme and then outputs the encoded sub-transmission data stream x3x4 to the antenna grouping unit 115. Then, the antenna grouping unit 115 receives the signals output from the STBC encoder #1 113-1 and the STBC encoder #2 113-2, performs antenna grouping based on antenna grouping matrix information, and transmits the antenna-grouped signals through corresponding transmission antennas during two symbol periods.
Since FIG. 1 corresponds to an example using the STBC scheme, the antenna grouping unit 115 performs antenna grouping and then transmits the antenna-grouped signals through corresponding transmission antennas during two symbol periods. However, in the case where the signal transmission apparatus uses a Space Frequency Block Coding (SFBC) scheme as well as the STBC scheme, the antenna grouping unit 115 may perform antenna grouping and then transmit the antenna-grouped signals through corresponding transmission antennas during one symbol period. For example, when the signal transmission apparatus uses an Orthogonal Frequency Division Multiplexing (OFDM) scheme, the antenna grouping unit 115 may, after performing the antenna grouping, transmit the antenna-grouped signals through corresponding transmission antennas and through two adjacent sub-carriers during one symbol period. Here, the antenna grouping matrix information is received through a receiver (not shown) from a signal reception apparatus corresponding to the signal transmission apparatus. The antenna grouping matrix information will be described below with reference to FIG. 2.
FIG. 2 is a block diagram illustrating a structure of a signal reception apparatus of a typical DSTTD mobile communication system.
Referring to FIG. 2, the signal reception apparatus includes reception antennas 211-1 and 211-2, a DSTTD receiver 213, and an antenna grouping matrix information generator 215. First, a signal received through the reception antennas 211-1 and 211-2 is input to the DSTTD receiver 213. Then, the DSTTD receiver 213 estimates the signal received through the reception antennas 211-1 and 211-2 as a transmission data stream {circumflex over (X)} by using a Vertical Bell Laboratories lAyered Space Time (V-BLAST) scheme based on a Minimum Mean Square Error (MMSE) scheme or a Zero Forcing (ZF) scheme.
Further, the DSTTD receiver 213 performs channel estimation for the signal received through the reception antennas 211-1 and 211-2, and then outputs a result of the channel estimation to the antenna grouping matrix information generator 215. The antenna grouping matrix information generator 215 generates antenna grouping matrix information in accordance with the channel estimation result output from the DSTTD receiver 213, and then transmits the generated antenna grouping matrix information to the signal transmission apparatus. Hereinafter, an operation of generating the antenna grouping matrix information by the antenna grouping matrix information generator 215 will be described.
The antenna grouping matrix information generated by the antenna grouping matrix information generator 215 include three bits, because the DSTTD mobile communication systems uses six antenna grouping matrixes. The antenna grouping matrix information generator 215 may select an antenna grouping matrix for minimizing the spatial correlation in accordance with the channel estimation result, an antenna grouping matrix for minimizing the Mean Square Error (MSE), or an antenna grouping matrix for maximizing the smaller Signal to Noise Ratio (SNR) from among the SNRs of the two sub-transmission data streams. The antenna grouping matrix can be defined by equation (1) below.
                              χ          0                =                  {                                                                                          [                                                                                                                        1                                                                                0                                                                                0                                                                                0                                                                                                                                0                                                                                1                                                                                0                                                                                0                                                                                                                                0                                                                                0                                                                                1                                                                                0                                                                                                                                0                                                                                0                                                                                0                                                                                1                                                                                                                      k                        =                        1                                                              ]                                    [                                                                                                              1                                                                          0                                                                          0                                                                          0                                                                                                                      0                                                                          1                                                                          0                                                                          0                                                                                                                      0                                                                          0                                                                          0                                                                          1                                                                                                                      0                                                                          0                                                                          1                                                                          0                                                                                                            k                      =                      2                                                        ]                                [                                                                                                    1                                                                    0                                                                    0                                                                    0                                                                                                            0                                                                    0                                                                    1                                                                    0                                                                                                            0                                                                    1                                                                    0                                                                    0                                                                                                            0                                                                    0                                                                    0                                                                    1                                                                                                  k                    =                    3                                                  ]                            [                                                                                          1                                                              0                                                              0                                                              0                                                                                                  0                                                              0                                                              0                                                              1                                                                                                  0                                                              1                                                              0                                                              0                                                                                                  0                                                              0                                                              1                                                              0                                                                                        k                  =                  4                                            ]                        ⁢                                                                           [                                                                                                              1                                                                          0                                                                          0                                                                          0                                                                                                                      0                                                                          0                                                                          1                                                                          0                                                                                                                      0                                                                          0                                                                          0                                                                          1                                                                                                                      0                                                                          1                                                                          0                                                                          0                                                                                                            k                      =                      5                                                        ]                                [                                                                                                    1                                                                    0                                                                    0                                                                    0                                                                                                            0                                                                    0                                                                    0                                                                    1                                                                                                            0                                                                    0                                                                    1                                                                    0                                                                                                            0                                                                    1                                                                    0                                                                    0                                                                                                  k                    =                    6                                                  ]                            }                                                          (        1        )            
In equation (1), χ0 refers to an antenna grouping matrix set, and k denotes an antenna grouping matrix index. Further, each row of each antenna grouping matrix is mapped to the transmission antenna index, and each column thereof is mapped to an input index of the antenna grouping unit 115. As noted from equation (1), each antenna grouping matrix included in the antenna grouping matrix set χ0 has a configuration in which every transmission antenna transmits one data stream. However, such a configuration of the antenna grouping matrix in which every transmission antenna transmits one data stream cannot solve the problem of spatial correlation of the transmission antennas.